1. Field of the Invention
The present invention relates to a printed circuit board.
2. Description of the Related Art
A rigid-flexible printed circuit board is in wide use in network equipment and mobile electronic products such as cell phones, etc. In mobile electronic products, in particular, there is a high demand for printed circuit boards having flexibility, since maneuverability is required of folders, sliders, or more complicated structures.
Compared to boards using electrical copper wiring, boards that use optical signals are unaffected by EMI and EMC, so that they may be more resistant to external noise and may not require the use of grounds or differential wiring. There may also be the advantage of enabling the transfer of high-speed signals, due to low loss.
An example of a conventional structure for embedding an optical waveguide in a flexible or rigid-flexible printed circuit board is as illustrated in FIG. 1A and FIG. 1B. Since it is effective to use copper wiring, even in a printed circuit board having an embedded optical waveguide, for transferring electrical power and low-speed analog signals, a board structure is generally presented in which electrical wiring and optical wiring are included together.
With the flexible or rigid-flexible printed circuit board, it may be required to provide sufficient bendability and high reliability in the bending portions. The bending characteristics may be determined by the thickness and strength of the flexible portions and the stiffness and strength of the material, while it may be especially desirable to keep the thickness small.
However, in the structure of a conventional printed circuit board having an embedded optical waveguide, the electrical wiring layer and the optical waveguide layer are attached, so that there is less bendability compared to typical electrical wiring boards, and there is a higher risk of defect or reliability problems.